Tokkai Hei 11-215881 published by the Japanese Patent Office in 1999 discloses a magnetic pole position detector of a brushless DC motor.
The detector is provided with a disk which rotates together with a rotor of the motor. The rotor is provided with four magnets so as to form four magnetic poles. The outer periphery of the disk is also magnetized to have four magnetic poles. Three Hall integrated circuits facing the outer periphery of the disk detect variations in a magnetic flux resulting from the rotation of the disk and output a signal indicating the rotation position of magnetic poles.